The present invention relates to a method for producing composite material, and, more particularly, relates to a method for producing composite material composed of a reinforcing material such as fiber, wire, powder, whiskers, or the like embedded within a matrix of metal.
There are known various types of reinforced materials, in which powder, whiskers, or fibers of a reinforcing material such as metal (stainless steel, for example), alumina, boron, carbon, or the like are embedded within a matrix of metal such as aluminum or magnesium or the like to form a composite material, and various methods of production for such composite or reinforced material have already been proposed.
One such known method for producing such fiber reinforced material is called the diffusion adhesion method, or the hot press method. In this method, a number of sheets are made of fiber and matrix metal by spraying molten matrix metal onto sheets or mats of fiber in a vacuum; and then these sheets are overlaid together, again in a vacuum, and are pressed together at high temperature so that they stick together by the matrix metal diffusing between them. This method has the disadvantage of requiring complicated manipulations to be undertaken in the inside of a vacuum device of a large size. This is clumsy, difficult, and expensive, and accordingly this diffusion adhesion method is unsuitable for mass production, due to high production cost and production time involved therein.
Another known method for producing such fiber reinforced material is called the infiltration soaking method, or the autoclave method. In this method, fiber is filled into a container, the fiber filled container is then evacuated of atmosphere, and then molten matrix metal is admitted into the container under pressure, so that this molten matrix metal infiltrates into the fiber within the container. Typically a fairly low pressure, such as 200 kg/cm.sup.2, is used. This method, also, requires the use of a vacuum device for producing a vacuum, in order to provide good contact between the matrix metal and the reinforcing material at their interface, without interference caused by atmospheric air trapped in the interstices of the fiber mass. In fact, if the combination of the reinforcing material and the matrix metal has poor wettability, a good resulting fiber reinforced material cannot be obtained. As a counter measure against this, it can be practiced, if the matrix metal is aluminum or an alloy thereof, to add a few percent of lithium to the matrix metal, so as to improve the wetting of the reinforcing fiber by the matrix metal; but this is expensive, due to the high cost of lithium. Yet further, this autoclave method also has the additional disadvantage that, if the molten matrix metal is magnesium, it is difficult to attain the required proper high degree of vacuum, due to the high vapor pressure of molten magnesium.
There is a further third method known for making fiber reinforced material, which does not use a vacuum device. In this method, the so called high pressure casting method, after charging a mold with reinforcing fiber, molten matrix metal is poured into the mold and is pressurized to a high pressure exceeding 1000 kg/cm.sup.2, and this high pressure forces the molten matrix metal to infiltrate into the interstices of the reinforcing material mass. Then the combination of the reinforcing material mass and the matrix metal is cooled down, while still being kept under this high pressure, until all the matrix metal has completely solidified.
This method has a certain degree of workability; but the difficulty arises that, since the temperature of the reinforcing material is less than the temperature of the molten matrix metal at the start of infiltration of the molten matrix metal into the interstices of the reinforcing material mass under pressure, this cools down the molten matrix metal, as it infiltrates into the reinforcing material mass, and causes it to at least partially solidify. Thereby, even when a high pressure like 1000 kg/cm.sup.2 is used, this infiltration pressure is insufficient, and it is found that the infiltration resistance of the reinforcing material mass to the molten matrix metal is too great. Accordingly, buckling of the reinforcing material mass, and change in the local density of the material thereof, occurs; and it is hard to obtain a resulting reinforced material of good and uniform composition and properties.
As a counter measure, it can be adopted to reduce the volume ratio of the reinforcing material mass, i.e. the proportion of its volume actually occupied by reinforcing material, to a low value such as 20% to 30%. However, it is generally desirable to use a greater volume ratio of reinforcing material than such a low ratio, from the point of view of obtaining desirable characteristics of the resulting composite material. Accordingly, this solution is not a welcome one, and is not suitable for general practice.
In this connection, two different methods have been used, in the prior art, for keeping the reinforcing material from being buckled and displaced, when the molten matrix metal is being infiltrated thereinto. One such method is to form the reinforcing material such as a fibrous mass into the shape of a mat, in advance; but this suffers from the limitation that the nature, density, shape, and fiber orientation of the reinforcing material are limited and constrained by the requirement that it should be formable into a mat. Another possibility has been to retain the reinforcing material in the desired shape and density by fitting the reinforcing material into a cavity formed in the casting mold, against the sides of the casting mold; but this solution suffers from the defect that, since the cavity retaining the reinforcing material is closely surrounded by the sides of the casting mold, and the molten matrix metal charged in the molding cavity is rapidly cooled, good composition of the reinforcing material and the matrix metal becomes difficult. Further, it can be quite hard to remove the resulting composite material from the casting mold, because it is in close proximity to the sides of the casting mold, if this method is used.
Sometimes, further, difficulties arise with regard to the thermal expansion of the reinforcing material, during the infiltration thereof by molten matrix metal, and the subsequent cooling. This can disturb the proper operation of any restraining means used for the reinforcing material mass.